1. Field of the Invention
The present invention relates generally to angular position and velocity sensors for devices having a limited angle of rotation, such as certain components of rotating computer memories and robot devices, and more particularly to a torsion transducer for use in conjunction with rotary actuator arms found in rotating computer memories.
2. Discussion of the Prior Art
Rigid magnetic disc drives having rotary actuator arms for positioning the read/write heads along an arc on the surface of the discs are usually powered by a voice coil motor of arcuate configuration, or by a stepper motor mechanically linked to the actuator arm. To make possible the accurate and rapid repositioning of the read/write heads over a desired circular track on the disc, or cylinder in the case of multiple discs, the drive motor of the rotary actuator arm must be carefully controlled. Voice coil motors are usually servo-controlled, wherein an accurate set of prerecorded circular concentric tracks, also called servo-tracks, on one or more of the disc surfaces provides feedback signals to the control circuitry through dedicated read/write heads. The principal function of servo tracks is to provide information for the precise positioning of the read/write heads over a predetermined data track. Additionally, the motion of the dedicated read/write head across the servo tracks while traveling to the desired position also provides signals that can be electronically processed to provide head position and velocity information which is then used to reposition the read/write head within the shortest possible time. A certain measure of this time is called access time. Another known means of obtaining position and velocity information, for the control of access time, is through the use of an encoder mounted between the rotary and the stationary portions of the actuator arm.
Typically, servo-controlled systems for a rotary actuator arm driven by a voice coil motor have the disadvantage of requiring at least one full disc surface dedicated to servo tracks, complex and expensive electronic control circuitry, and in some cases, expensive additional position and velocity sensing means, to provide adequate access time for the disc drive. A significant portion of the access time in such systems is the settling time required upon reaching a desired track position.
In the case where the rotary actuator arm is driven by a stepper motor, the re-positioning of the read/write heads is accomplished by control circuitry that keeps count of the steps travelled by the motor, while the settling time is managed by complex algorithms that control the end-point positioning of the stepper motor shaft. Such systems do not generally use servo-tracks on the discs and thus do not require servo-writing equipment or electronic control circuitry on the drive, nor do they give up space on the disc surface, that would otherwise be available for the storage of data. However, stepper motor driven rotary actuator arms have some limitations on access time due to the absence of feedback signals.